Relatively light hydrocarbon liquids (crude oil, natural gas condensate (hereinafter, referred to as condensate), natural gas, and the like) produced from wells and exhaust combustion gas sometimes contain mercury or a mercury compound. However, in plants for natural gas processing, petroleum refining, and petrochemistry, since mercury contained in a hydrocarbon liquid is likely to corrode apparatuses made of aluminum-based materials and to poison precious metal catalysts, it is necessary to lower the mercury concentration in a processed liquid to a very low value.
Hydrocarbon liquids contain various kinds of mercury such as ionic mercury (for example, “RS—Hg—SR” and the like) and organic mercury (for example, “R—Hg—R” and the like”) as well as elemental mercury, and actually, ionic mercury is contained in many cases.
The simplest and most realistic method for removing mercury contained in a hydrocarbon liquid is adsorptive removal using mercury-removal adsorbent. However, the use of this method for removing mercury contained in a hydrocarbon liquid has the following problems: (1) ionic mercury and organic mercury have far lower adsorptivity to mercury-removal adsorbent than elemental mercury; (2) A hydrocarbon liquid contains heavy hydrocarbon which lowers adsorptivity of mercury; and (3) A hydrocarbon liquid contains a third component exhibiting very strong adsorptivity to mercury-removal adsorbent, and the strong adsorptivity of the third component often hinders the originally intended mercury adsorption. In such cases, a conventional adsorption method using metal sulfide (molybdenum sulfide, copper sulfide, or the like) as mercury-removal adsorbent has a great difficulty in lowering the mercury concentration to a very low value on the 1 wtppb order. Lowering the concentration, even if possible, is not practical due to a short life of an adsorption tower. An adsorption method capable of removing such adsorption-resistant ionic mercury and the like would have a great commercial value.
As attempts to thus removing mercury by adsorption in liquid hydrocarbon, there have been proposed mercury-removal adsorbents (patent document 1) including activated carbon carrying metal halide such as potassium iodide in an amount of 0.5 to 25 wt % of the total adsorbent weight, and mercury-removal adsorbents (patent document 2) using activated carbon with a 50 to 70% water retention rate, thereby setting an amount of metal halide carried to 25 wt % or more of the total adsorbent weight.
In general, to avoid the aforesaid corrosion of the aluminum-based materials and poisoning of the precious metal catalysts, the adsorptive removal need to lower the concentration of mercury in liquid hydrocarbon to 1 wtppb or lower. However, as a result of tests, neither of the aforesaid mercury-removal adsorbents was able to exhibit results satisfying the specification value, that is, the mercury concentration of 1 wtppb or lower in liquid hydrocarbon. Incidentally, the patent document 1 reports that its mercury-removal adsorbents were able to lower the mercury concentration in hydrocarbon to less than 3 wtppb which is the lowest detection limit of mercury in this test, but this does not guarantee the aforesaid specification value at all.
Patent Document 1
Japanese Patent Publication No. 2602361: Table IV, Table V
Patent Document 2
Japanese Patent Application Laid-open No. 2005-349364; Table 1